Upstream oil field exploration and production operations, including drilling, completion, stimulation, water injection, and other liquid and gas injection processes, often require using chemical and solid materials to temporarily seal off very high permeability rocks (commonly called “thief zones”). In drilling and completion, sealing off thief zones prevents lost circulation of drilling and completion fluids. In stimulation and other production and recovery enhancing processes, stopping loss to thief zones improves the uniformity of the injected stimulation fluid entering the formations along the trajectory of the wellbore, leading to improved hydrocarbon production and recovery. Efficient plugging of the thief zones optimizes the short term and maximizes the long term economy of the hydrocarbon extraction from the subterranean formations. If not properly designed, however, the materials injected into the well could fail to seal the thief zones, or, worse, they could damage the formation permeability such that the reservoir could become non-productive.
To create strong and temporary plugs, viscous fluids containing degradable fibers and/or solid particles are mixed by surface equipment prior to their injection. The fibers and/or particles are expected to interact with one another and form flocs and/or clusters to plug high permeability zones and fractures. The fiber flocculation and the particle coagulation are known to be highly sensitive to the viscosity of the carrying fluid, the injection rate, and the concentrations of certain additives. The quality control of the resulting mixture is done by visual inspection on the surface. However, when injected into the formation, the operator has no indication about the interaction of the dispersed phase, even though it is a very important condition for efficient placement of the treatment fluid. Indeed, an early flocculation/coagulation will plug surface or downhole equipment and a late one will result in an unsuccessful fluid placement.
There currently are two primary methods to inject fibers downhole to reduce or eliminate fluid loss to a thief zone. One, called “batch” mixing and shown in FIG. 1, mixes the fibers with the base fluid prior to injection using an appropriate mixer and then injects the mixture into the well with a high pressure pump. The other method, known as “on-the-fly” mixing and shown in FIG. 2, mixes while performing the fluid injection by feeding the fiber into the flow stream, by using a venturi to suck the fiber from the feeder into the flow stream tubing.